The present invention relates to a method for preventing polymer scale deposition on the reactor walls in the polymerization of a vinylic monomer such as vinyl chloride.
Vinylic monomers are polymerized in several types of polymerization procedures including suspesnion polymerization, emulsion polymerization, solution polymerization, gas-phase polymerization and bulk polymerization. One of the difficult problems common to all of these polymerization procedures in the prior art is the deposition of polymer scale on the reactor walls and the surfaces of other equipments such as the stirrer and the like coming into contact with the monomer under polymerization.
That is, when a vinylic monomer is polymerized in either one of the above mentioned polymerization procedures, the polymer is deposited more or less on the walls of the reactor and the surfaces of the stirrer and other equipments coming into contact with the monomer to form scale thereon so that various disadvantages are unavoidable such as the decrease in the yield of the desired polymer and lowered cooling capacity of the polymerization reactor as well as degraded quality of the polymer product due to the polymer scale eventually entering the polymer product as coming off the reactor walls. In addition, such polymer scale must be removed taking a great deal of time and labor in order to prepare the polymerization reactor for the next run of the polymerization. The scale removing works by man power present a very serious problem of safety against human body because the polymer scale usually contains a considerably large amount of the unreacted monomer absorbed therein which is sometimes toxic as is a matter of very serious concern in recent years as in the case of vinyl chloride in particular.
A number of methods have been proposed hitherto to prevent the polymer scale deposition on the reactor walls. Exemplary of such methods are a method in which the reactor walls and the surfaces of the stirrer and the like are coated, prior to the polymerization, with a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds and the like or a dye or pigment (see, for example, Japanese Patent Publication Nos. 45-30343 and 45-30835), a method in which the walls and surfaces are coated with a polar organic compound or a dye having been treated with a metal salt (see Japanese Patent Publication No. 52-24953), a method in which the coating material is a mixture of an electron donor compound and an electron acceptor compound (see Japanese Patent Publication No. 53-28347) and a method in which the coating material is an inorganic salt or an inorganic complex compound (see Japanese Patent Publication No. 52-24070).
The above mentioned methods by coating are indeed effective in preventing polymer scale deposition on the thus coated surfaces when the polymerization initator used in the reaction is an azo compound or an organic peroxide having a long-chain alkyl group. The effect of preventing polymer scale deposition by these coating methods is greatly decreased or hardly obtained, for example, in the suspension polymerization of vinyl chloride in an aqueous polymerization medium in which the polymerization initiator is an organic peroxide which is soluble in the monomer but has a relatively large solubility in water of 0.2% by weight or larger at 20.degree. C. In addition, effectiveness of such a coating method for polymer scale prevention can hardly be expected when the polymerization procedure is for the preparation of a polymer of styrene, copolymers of styrene and butadiene, copolymers of acrylonitrile, butadiene and styrene and the like.
Accordingly, whereas the use of an organic peroxide soluble in monomer but having a solubility in water of 0.2% by weight or larger at 20.degree. C. as the polymerization initiator is highly desirable, in particular, in the suspension polymerization of vinyl chloride due to the superior quality of the polyvinyl chloride products in respects of little initial coloring of the polymer in the course of fabrication by molding and high heat stability of the shaped articles, such a polymerization initiator can hardly be put to practical use due to the difficulty in preventing the polymer scale deposition on the reactor walls. Therefore, it has been an important technical problem to be solved to establish an effective method for the polymer scale prevention even in the suspension polymerization of vinyl chloride using such an organic peroxide as mentioned above as the polymerization initiator.